As well known in the art, the hydraulic pressure is used to transmit power or information about an operating condition. For example, hydraulic pressure established by a pump is delivered to a motor to propel a vehicle or to carry a heavy load. In addition, a pressure level of the hydraulic pressure to be delivered to an actuator such as a motor is regulated, and the hydraulic pressure is also used as a control signal for permitting and blocking a delivery of the hydraulic fluid.
One example of a hydraulic circuit of this kind is described in the European Patent No. EP0985855. A belt-driven continuously variable transmission is employed in the vehicle disclosed in the European Patent No. EP0985855. According to the teachings of the European Patent No. EP0985855, a hydraulic circuit is comprised of a motor driven oil pump and an accumulator for storing hydraulic fluid delivered from the oil pump. A speed ratio and a clamping pressure of the continuously variable transmission are controlled by controlling the hydraulic pressure applied to hydraulic chambers of a primary pulley and a secondary pulley from the oil pump and the accumulator. For this purpose, delivery of the hydraulic fluid to the primary and secondary pulleys is controlled using electrically-controlled solenoid valves. Specifically, when the feeding solenoid valve is energized to be opened, the hydraulic fluid is allowed to be delivered to the primary pulley thereby decreasing a speed ratio, and also allowed to be delivered to the secondary pulley thereby increasing a clamping pressure. In contrast, when the draining solenoid valve is energized to be opened, the hydraulic fluid is withdrawn from the primary pulley thereby increasing a speed ratio, and also withdrawn from the secondary pulley thereby decreasing a clamping pressure. The speed ratio and the clamping pressure thus controlled are held constant by closing all of the solenoid valves to confine the hydraulic fluid in the hydraulic chambers of the primary pulley and the secondary pulley.
The oil to be delivered to the vehicle transmission is used not only to transmit a power or acting force but also used for a lubrication purpose. The lubrication oil is delivered to a frictional contact surface and forms an oil film thereon. Therefore, contaminants such as metallic dust, carbon dust etc. may be mixed with the lubricating oil. For example, Japanese Patent Laid-Open No. 2008-144830 discloses a solution to avoid such an event by delivering the lubricating oil to a toroidal continuously variable transmission through a filter. Japanese Patent Laid-Open No. H05-184212 also discloses a structure configured to such oil into the oil pump through a filter.
Thus, in the system using the oil to control the actuators and to lubricate a site of friction, a filter is conventionally employed to remove the contaminants from the oil thereby avoiding clogging of valves and oil passages. Therefore, in the hydraulic control system for the belt-driven continuously variable transmission taught by the European Patent No. EP0985855, such a filter is also disposed at a suction inlet of the motor-driven oil pump to remove the contaminants from the oil sucked from an oil pan.
It is possible to capture the contaminants certainly by using a fine filter. In case of using the fine filter, however, the filter may become quickly clogged with the captured contaminant thereby increasing resistance of the oil to flow through the filter. When confining pressure within the hydraulic chambers of the primary and the secondary pulleys by closing the solenoid valves as described in the European Patent No. EP0985855, such fine filter may prevent contaminants from getting stuck between a valve seat and a valve element of the solenoid valve so that the pressure can be confined within the hydraulic chambers certainly. However, such fine filter will increase resistance to pump up the oil, and this will result in an increase in a power loss. In addition, the oil pump has to be continuously driven to deliver the oil not only to the hydraulic chambers of the primary and the secondary pulleys for the purpose of controlling the speed ratio and the clamping pressure of the belt-driven CVT, but also to lubrication points such as bearings, clutches, gears etc. In this situation, therefore, the power loss will be further increased.